An effective measure is desired to prevent automobile paint films from suffering damages, dulling, discoloration, and other defects due to various suspended or impinging matters such as dust particles and raindrops. Such paint film protection is necessary, for example, in the case of loading painted automobiles or automotive parts into trucks or ships and transporting these to remote places such as overseas.
A method of covering painted automobiles or automotive parts with a 5-20 μm-thick coating comprising a wax has been known as a measure for such paint film protection. However, this method has had, for example, the following various problems. It is difficult to form a wax coating at a uniform thickness, so that uniform protection of the paint film cannot be obtained; wax coatings are prone to be fouled and have poor resistance to acid rain; part of the wax may penetrate into the paint film to cause discoloration or other defects; formation of a wax coating and removal thereof require much labor; and use of solvents and treatment of waste liquids are apt to lead to environmental problems.
On the other hand, various kinds of surface-protective sheets are known which comprise a substrate and a pressure-sensitive adhesive layer formed thereon. Of these, a protective sheet comprising a radiation-curable pressure-sensitive adhesive layer having a lowered glass transition point has been proposed for use on adherends having a paint film (JP-A-2-199184). (The term “JP-A” as used herein means an “unexamined published Japanese patent application”.)
The above surface-protective sheet, however, has been defective in that preparation of a radiation-curable pressure-sensitive adhesive used to form the pressure-sensitive adhesive layer requires a complicated and special technique. In addition, the protective sheet has had other problems that the pressure-sensitive adhesive layer obtained shows poor adhesion strength and that the pressure-sensitive adhesive layer has poor heat resistance, probably because the adhesive layer has been made to have a lowered glass transition point to impart a particular performance required in paint film-protective sheets, i.e., preventing the deformation of the paint film. Due to such a pressure-sensitive adhesive layer, application of the protective sheet to automobiles has had a problem that the adhesion strength of the sheet applied to an automobile increases with increasing temperature during outdoor transportation, etc., making stripping of the sheet difficult. Thus, the proposed surface-protective sheet has been unsatisfactory in basic performance.